For high-speed optical communications of 10 Gbps or greater, an optical modulator that includes Mach-Zehnder modulators is often used. The optical modulator includes two Mach-Zehnder modulators (an I-arm Mach-Zehnder modulator and a Q-arm Mach-Zehnder modulator). Each Mach-Zehnder modulator generates an optical signal (an I-arm optical signal and a Q-arm optical signal) based on a drive signal. The optical modulator includes a phase shifter to provide a phase difference of π/2 between the I-arm optical signal and the Q-arm optical signal. The optical modulator generates a modulated optical signal by combining the I-arm optical signal and the Q-arm optical signal.
To generate a high-quality modulated optical signal, the phase difference generated by the phase shifter needs to be accurately controlled to be π/2. Accordingly, methods have been proposed for control of a phase difference generated by a phase shifter. For example, in dithering-based feedback control, low frequency signals are respectively given to an I arm and a Q arm. Then, a DC bias voltage applied to a phase shifter is controlled according to a low frequency signal component contained in output light of an optical modulator. Such feedback control is described by, for example, Japanese Laid-open Patent Publication No. 2007-133176, International Publication Pamphlet No. WO2011/104838 (Japanese Patent No. 5318278), and International Publication Pamphlet No. WO2011/030763 (Japanese Patent No. 5261779).
In many cases, an optical module that includes an optical transceiver with digital coherent technique is required to transmit and receive an optical signal in a desired wavelength channel. In this case, the optical module includes a wavelength-tunable light source to generate continuous wave light of a specified wavelength. When data is transmitted under a modulating scheme in which the number of bits per symbol is large, the optical intensity of a transmission signal may be decreased due to a modulation loss. Meanwhile, narrowing the bandwidth of each wavelength channel so as to enhance the efficiency of frequency utilization may decrease the optical intensity of the transmission signal. Accordingly, a modulated optical signal generated by an optical modulator is amplified by an optical amplifier. However, amplifying an optical signal using an optical amplifier also amplifies noise, and this could degrade an optical SNR (Signal-to-Noise Ratio). Accordingly, an optical module includes a wavelength tunable filter to transmit or receive a specified wavelength channel.
The wavelength of a wavelength tunable light source is controlled through dithering. In addition, the center wavelength of a passband of the wavelength tunable filter is also controlled through dithering. That is, the bias of the optical modulator, the wavelength of the wavelength tunable light source, and the center wavelength of the passband of the wavelength tunable filter are all controlled using a low frequency signal. In addition, in the prior arts (e.g., the configurations described in the patent documents described above), a plurality of low frequency signals with different frequencies are used to control the bias of the phase shifter of an optical modulator.
However, to control those signals in parallel, the frequencies of the individual low frequency signals need to be different from each other. Hence, in a configuration in which only a limited frequency band can be used for control in an optical module, it is preferable that few frequencies be used to control the DC bias of the phase shifter of an optical modulator.